Odoo POS does not support this behavior out-of-the-box , so to achieve your goal — making partial refunds consider the proportional coupon discount — you need to customize both the POS frontend (JavaScript) and optionally the backend (Python) if you're using accounting or syncing with invoices.

Summary of the Required Customization

When a coupon or global discount is applied on a POS order:

• Each product line should store the proportional share of the discount .
• If a product is refunded, the refund amount should match the discounted value , not the full price.

HOW TO CUSTOMIZE — STEP BY STEP

STEP 1: Extend POS Order Line to Store Proportional Discount

In the POS frontend (JS):

• Modify the logic when a coupon/global discount is applied .
• Calculate each line's discount share and store it.

Example:

// Extend OrderLine model
const OrderlineSuper = Orderline.prototype;
Orderline = Orderline.extend({
    set_discounted_price: function(discounted_price) {
        this.discounted_price = discounted_price;
    },
    get_discounted_price: function() {
        return this.discounted_price || this.get_unit_price();
    },
});

When applying the coupon, distribute the discount proportionally:

const total = order.get_total_without_tax(); // eg 4295.0
const discount_value = total * 0.10; // 429.5
order.get_orderlines().forEach(function(line) {
    const line_value = line.get_price_without_tax();
    const proportional_discount = (line_value / total) * discount_value;
    const discounted_price = line.get_unit_price() - (proportional_discount / line.get_quantity());
    line.set_discounted_price(discounted_price);
});

STEP 2: Modify the Refund Process to Use Discounted Price

When refunding:

In the ReturnProductScreen , before adding a return line:

const discounted_price = original_line.get_discounted_price();
refund_line.set_unit_price(discounted_price);

Thanks & Regards,

Email: contact@datainteger.com